The present invention relates to pump motor control circuitry and more particularly to improved circuitry for compensating for pulsations in pump output flow in chromatographic systems.
Chromatography is a technique for separating a mixture of components, known as a sample, by distributing the sample in dynamic equilibrium between two phases in ratios characteristic of each component. The sample is normally dissolved in a flowing mobile phase and forced through a stationary phase, as by pumping, to cause each component of the sample to migrate through the stationary phase at a characteristic rate. After a period of time, the migration results in the separation of the components into individual zones which can be detected by a detector to identify individual components.
In order to provide for resolution of the components within the sample, various characteristics of the system (e.g., flow-rate, chemical composition, temperature) can be changed to improve system performance. Of particular importance is the ability to maintain a relatively pulse free pump output flow for any particular system flow-rate. In operation, a system may be controlled to provide a constant or variable flow-rate with a single pump, constant flow-rate with a programmed chemical composition in a gradient system, a constant chemical composition with a programmed flow-rate in a gradient system, or a programmed flow-rate and chemical composition in a gradient system. In each case, however, the sensitivity of detection and quantitation of the zones depends upon the noise level of the detector. Since detector noise is aggravated by pulsations in the flow-rate, the sensitivity of analysis, and thus the resolution and reproducibility of system performance depends on the capability of maintaining the pump output flow relatively free of pulsations caused by pump refill and repressurization over the operating range of flow-rates and pressures for the chromatographic system.
In the prior art various techniques have been proposed to minimize or eliminate pulsations in pump output flow. In one such system, pump speed is increased during refill and repressurization and pump pressure, detected as a function of motor torque, is used to produce a signal for controlling the length of the piston stroke through which the motor is speeded up for rapid repressurization. While in this arrangement some compensation is provided, the control fails to provide sufficient compensation over the desired range of flow-rates and pressures in a chromatographic system. Such system fails to provide tracking for repressurization as a function of varying flow-rates. Thus, if the compensation circuit is set to minimize pump flow pulsations at low flow-rates, the pulsations will be under-compensated at high flow-rates and high pressures. More specifically, as the flow-rate setting is increased, the average output flow-rate actually drops off at high flow-rates since the time interval for actual physical repressurization of the pump, as determined by the prior art circuitry, tends to become a larger fraction of the pumping cycle as the flow-rate increases. In addition, since the repressurization signal is derived from a measure of motor torque rather than actual pressure, pulse compensation does not accurately track system pressure.
In the noted prior art system, the detected motor torque is also used to indicate fault conditions due to over and under pressures at which the chromatographic system will not properly operate. Such system, however, provides circuitry which does not enable the setting of accurate reference limits to control alarm and pump shutdown during fault conditions. In addition, since the motor torque is used as a measure of pressure, the system may respond to torque conditions causing alarm and pump shutdown which are not actual pressure faults. In still other instances, the circuitry is not capable of detecting pressure fault conditions that should signify alarm and pump shutdown.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide pump control circuitry for allowing improved pulse compensation and fault detection in chromatographic systems.